diff --git a/simgear/props/props.hxx b/simgear/props/props.hxx
index 12d7ba79..41361c98 100644
--- a/simgear/props/props.hxx
+++ b/simgear/props/props.hxx
@@ -1110,6 +1110,12 @@ public:
   void removeChangeListener (SGPropertyChangeListener * listener);
 
 
+  /**
+   * Get the number of listeners.
+   */
+  int nListeners () const { return _listeners ? _listeners->size() : 0; }
+
+
   /**
    * Fire a value change event to all listeners.
    */